Variable gas leak rate valves are used for precise control of gas admission to various laboratory and industrial instruments or systems, including by way of example, accelerators, spectrometers and surface and plasma physics apparatus. Prior valves utilized for this purpose often relied on a seal produced mechanically between two solid pieces using such as a tapered fit or a deformable soft metal ring-seal in which the sealing members are precisely moved with respect to each other. These schemes suffer from a lack of repeatability or smooth control over orders of magnitude of leak rate. Other valves relied on the leakage produced between two solid materials having different thermal expansions by heating the materials to provide a wide range control of leak rates but required very large amounts of electrical power to operate and suffered from corrosion when reactive gases flow between the heated valve members.